The aim of this research program is to study the erythrocyte population change in growing newborn animals. Emphasis will be placed in the study of the transition from fetal to adult characteristics of membrane transport properties of red blood cells in the newborns of various mammalian species which have red cells of higher or low potassium concentration. Attempts will be made to separate for the purpose of this study fetal red cells and neonatal or adult cells from the same individual newborn by a density gradient centrifugation technique. Particular attention will be focused on the characterization of the kinetics of K and Na transport in fetal and adult red cells and also in maturing red cells in vivo. It is hoped that information gained from this investigation will contribute not only to the understanding of red cells population change in newborn animals but also to the understanding of the maturation of membrane function for transport processes.